(1) Field of the Invention
The present invention relates to a content usage management system which comprises a server for distributing a digital content such as video and music data and a client apparatus, a content usage management method and a client apparatus.
(2) Description of the Prior Art
In a content distribution system via a network, a content to be distributed is encrypted and the encrypted content and a decryption key for the encrypted content (hereafter referred to as content key) is distributed to a user terminal.
As for a distribution method of a content key, two types of distribution methods exist: a unicast distribution which distributes a content key one on one to only a client apparatus that requests the content use; and a multicast distribution which distributes a content key to all client apparatuses at once even when they do not request the content use. In a conventional content distribution system, a content key is distributed by the same distribution method regardless of the types of contents.
The above description is disclosed in a patent literature, Japanese Laid-Open application No. 11-312175.
However, the number of clients to which a content key is distributed differs depending on contents. If the number of client apparatuses which use the content is large, the multicast distribution reduces processing loads on a server than the unicast distribution since the unicast distribution distributes a content key after communicating with each client apparatus. On the other hand, the unicast distribution can ensure higher security than the multicast distribution to protect copyrights since the unicast distribution can keep in a log information about which content keys were sent to which client apparatuses. In the conventional technologies, even if sending a content key by using the multicast distribution can reduce the processing load on a server depending on contents, the method which puts more processing loads on a server is actually used to distribute a content key because a content key is distributed by either one of the distribution methods regardless of content types. It is also distributed by the method which provides lower security even if higher security is desired depending on contents.